1. Field of the Invention
The present invention relates to a slab waveguide comprising a photonic crystal, having a refractive index distribution in the film thickness direction, and used in an optical planar circuit. The present invention also relates to a method of manufacturing the slab waveguide.
2. Related Art of the Invention
FIGS. 21(a) and 21(b) show slab waveguides with a substrate, which are examples of a first conventional slab waveguide, and each of which is constituted by a photonic crystal.
The slab waveguide shown in FIG. 21(a) has a substrate 201 and a slab-type photonic crystal 200 forming a slab 203 on the substrate 201. Cylindrical vacancies 202 are formed in the slab 203. The vacancies 202 extend in the thickness direction of the slab 203 and are two-dimensionally and periodically arranged parallel to the substrate 201. The slab 203 is uniform in refractive index. The refractive index of the slab 203 is larger than that of the substrate 201. The thus-formed slab-type photonic crystal 200 used as a slab waveguide is capable of reducing the speed of light propagating in the slab waveguide, dispersing the wavelength of light, or deflecting the direction of traveling of light.
Propagation of light in a photonic crystal has been known by using as the refractive index of a photonic crystal the effective refractive index defined as the volumetric ratio of the refractive indices of a plurality of materials periodically arranged. Such macroscopic use is effective in a case where the refractive index period is sufficiently smaller than the wavelength of light, because light behaves according to the average of refractive indices. In a case where the refractive index period is close to the wavelength of light, however, light behaves according to each of refractive indices and, therefore, it is necessary to make microscopic use such as to treat each of different refractive index materials periodically arranged.
Actually, in a macroscopic use, it is contemplated that if a substrate 201 having a refractive index lower than the effective refractive index obtained by averaging the refractive index of vacancies 202 and the refractive index of the slab 203 is used, light propagates through the above-described slab-type photonic crystal 200. However, according to a microscopic use, light 204 incident upon the slab-type photonic crystal 200 in the slab waveguide with such a substrate propagates through a slab 203 portion periodically formed and having a refractive index higher than that of the substrate 201, but diffused light 206 in the vacancy portions 202 leaks to the substrate 201 side because the reflective index of the vacancies 202 is lower than that of substrate 201, and only part of diffused light 206 can propagates through the vacancies 202. In this case, the amount of light 205 emergent from the above-described slab-type photonic crystal 200 is substantially zero.
Also in the case of a slab waveguide with a substrate in which, as shown in FIG. 21(b), substrate vacancies 207 are provided by extending the vacancies 202 in the substrate 201 to reduce the effective refractive index of the substrate 201 portion, light does not propagate through the slab-type photonic crystal 200.
FIGS. 21(c) and 21(d) show slab waveguides with no substrate, which are examples of a second conventional slab waveguide, and each of which is constituted by a photonic crystal.
Each of the slab waveguides shown in FIGS. 21(a) and 21(b) comprises a slab-type photonic crystal 200. The slab-type photonic crystals 200 of these slab waveguides are formed in the same manner except that they differ in thickness from each other. In each slab-type photonic crystals 200, cylindrical vacancies 202 are formed. The vacancies 202 extend in the thickness direction of the slab 203 and are two-dimensionally and periodically arranged parallel to the major surfaces of the slab 203. The slab 203 is uniform in refractive index.
In a case where a slab waveguide comprising only the above-described slab photonic crystal 200 with no substrate, the slab photonic crystal 200 behaves like a lens waveguide, slab portions having a higher refractive index act as a lens, and leakage by diffusion does not occur in the vacancies 202 having a lower refractive index. As a result, incident light 204 propagates in the slab-type photonic crystal 200 without diffusing.
If different film thicknesses such as shown in FIGS. 21(c) and 21(d) are set, different states of propagation of light are exhibited. In a case where the film thickness shown in FIG. 21(c) is about several microns or less, light is reflected at the boundary in the slab thickness direction between air and the slab 203 portion having a higher refractive index so as to propagate in a light multimode propagation manner. To satisfy single-mode conditions, therefore, it is necessary to set the slab thickness to 1 μm or less. In this case, there is no problem with propagation in the slab-type photonic crystal 200, but the coupling loss in coupling to an optical fiber having a core diameter of about 8 μm is large because of a difference in mode field diameter from the optical fiber.
On the other hand, in a case where the film thickness shown in FIG. 21(d) is large, about 10 μm, light is not reflected at the boundary in the slab thickness direction between air and the slab 203 portion having a higher refractive index. In this case, therefore, the slab waveguide is formed as an ideal lens waveguide and the propagation of light is single-mode propagation. Also, since there is no difference in mode field from the optical fiber, the coupling loss in coupling to the optical fiber is small. In this case, however, it is necessary to make vacancies having a period close to that of light with respect to a film thickness of 10 μm or more, i.e., an aspect ratio of 50 or more. At the present time, it is extremely difficult to realize such a high aspect ratio.
In either of the cases shown in FIGS. 21(c) and 21(d), there is a need to reinforce the slab by some means in putting the slab waveguide to practical use and there is also a need to achieve a device design by considering use of a member corresponding to a substrate.
The invention disclosed in Japanese Patent Laid-open No. 2001-337236 is equivalent to an arrangement in which the vacancies 202 shown in FIG. 21(a) are filled with a material having a refractive index lower than that of the slab 203, and in which the upper clad layer in an upper portion of the slab and a lower clad layer (substrate) 201 are formed of a material having a refractive index lower than that of the material filling the vacancies. In this arrangement, light is totally reflected at the clad layer boundary as seen from a microscopic viewpoint and, accordingly, light is guided through the slab. In actuality, however, no material having a refractive index as low as that of air exists. Therefore, it is not possible to set between the slab and vacancies a refractive index difference sufficient for enabling the photonic crystal to operate sufficiently effectively.
In the case of slab-type photonic crystals such as those in the above-described conventional arts in which vacancies are periodically formed in a slab uniform in refractive index, it is difficult to satisfy all of conditions (1) to (3) shown below.    (1) The mode field diameter is close to that of an optical fiber.    (2) Single-mode conditions are satisfied, as are those in the case of an ideal lens waveguide.    (3) The slab waveguide has a strength such as to be capable of being put to practical use.
In the arrangements according to conventional methods, the refractive index of the substrate is higher than that of the vacancies in the photonic crystal and, therefore, leakage of light from the vacancies occurs and light cannot propagate. The arrangement using no substrate and free from leakage of light entails, for satisfaction of single-mode conditions, the need to set the film thickness of the slab to a small value of 1 μm or less or to an increased value of 10 μtm or greater. If the film thickness of the slab is reduced, coupling to an optical fiber is difficult. If the film thickness of the slab is increased, it is difficult to fabricate the slab waveguide. Moreover, the strength of the slab in a single state is so low that the slab waveguide is incapable of being put to practical use.